1. Field of the Invention
The present invention relates to a method of and an apparatus for a charge-free radical or down-flow type plasma processing, such as plasma etching and plasma ashing, in which a reactant source gas is activated by a high frequency electromagnetic wave, such as microwaves and radio frequency waves, to form a plasma and thereby to generate chemically active species or radicals which react with a wafer, such as a silicon wafer, to perform "etching" or "ashing" of the wafer, while the plasma itself is shielded from the wafer by a shielding plate.
2. Description of the Related Art
One of the important aspects of plasma processing, such as etching or ashing of a silicon wafer or the like is the distribution of the rate of processing over the wafer surface. As the wafer size increases, a uniform distribution of the processing rate must be maintained over a larger area of the wafer. To this end, the pressure and flow of the source gas must be precisely controlled in the vicinity of the wafer surface such that the active species spreads uniformly over the wafer surface.
FIG. 1 schematically illustrates an arrangement of a conventional plasma processing apparatus, such as plasma etcher or asher, as disclosed in T. Tsuchiya et al., "Proceedings of the Symposia on Reliability of Semiconductor Devices and Interconnection and Multilevel Metallization, Interconnection, and Contact Technologies", Vol. 89-6 (May 7, 1989), pp.425-435. The plasm etcher as disclosed therein comprising a reactant gas supply pipe 51, a plasma generating chamber 52, a microwave transmitting window 53 of quartz or ceramics, a waveguide 54, a plasma outlet 55, a plasma shielding plate 56, a processing (etching or ashing) chamber 57, a wafer stage 58, a wafer 59, and an exhaust port 60.
In the conventional arrangement as shown in FIG. 1, plasma processing such as etching or ashing of the wafer 59 is performed by mounting the wafer 59, such as a silicon wafer on the wafer, stage 58, supplying a reactant source gas (such as a mixed CF.sub.4 +O.sub.2 gas or other etching gas when performing etching) through the reactant gas supply pipe 51 to the plasma generating chamber 52, and 10 activating there the reactant gas by a microwave from a microwave generator (not shown), guided along the waveguide 54 and introduced through the microwave transmitting window 53, to form a plasma in the plasma generating chamber 52, the plasma being allowed to flow into the etching or ashing chamber 57 through the plasma outlet 55.
While passing through a gap, along the backside of the shielding plate 56, the plasma flowing from the plasma generating chamber 52 is neutralized to form an active species or radical, which reacts with the wafer 59 mounted on the wafer stage 58, thereby etching or ashing the wafer surface. Thereafter, the gas is exhausted through the exhaust port 60.
In the conventional arrangement as shown in FIG. 1, the level of the wafer stage 58 is adjusted by trial and error to provide an optimum distribution of the processing rate (i.e. etching or ashing rate) over the wafer surface.
An optimum distribution of the processing rate over the wafer surface is obtained by actually processing the wafer surface at different levels of the wafer stage.
The adjustment of the wafer stage level alone, however, does not provide a uniform bombardment of the active species against the wafer surface, but only provides a limited processing rate distribution over the wafer surface of about 3-5% at best.
Therefore, a reduced yield in the production of semiconductor devices is observed when using such a wafer.